Barbara Liskov wanted to explore abstract data types and started to design the CLU programming language in 1973, as part of the Computation Structures Group of Project MAC. The computer hosting development was the Dynamic Modeling PDP-10 running ITS. Initial work was done using Maclisp, but was moved over to the Muddle language which was integral to the Dynamic Modeling project. Later versions would be increasingly self-hosting. As the DM PDP-10 was getting older, CLU moved to the MIt-XX TOPS-20 machine, and later Unix on VAX and Sun machines. There is now a Portable CLU for Linux etc.
During my ITS restoration work, I encountered files related to CLU. I contacted Liskov and she approved their release. MIT agreed to put then online here: GitHub - MITDDC/clu-1976-1989: A set of files that can be used to create a working version of an e
I added the most recent 1978 version of the CLU compiler and runtime to the ITS repository and added a “hello world” example: its/hello.clu at f29fbb16a13af66fe88d524cb6faf0e87257f44d · PDP-10/its · GitHub